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We consider propagation of surface plasmon polaritons (SPPs) in linearperiodic chains (LPCs) of prolate and oblate metallic spheroids. We show thatthe SPP group velocity can be efficiently controlled by varying the aspectratio of the spheroids. For sufficiently small aspect ratios, a gap appears inthe first Brillouin zone of the chain lattice in which propagating modes do notexist. depending on the SPP polarization, the gap extends to certain intervalsof the Bloch wave number q. Thus, for transverse polarization, no propagatingSPPs exist with wave numbers q such that q_c^\perp < | q | < \pi/h, h being thechain period. For longitudinally polarized SPPs, the gap spans the interval|q|<q_c^\parallel. Here q_c^\perp and q_c^\parallel are different constantswhich depend on the chain parameters, spheroid aspect ratio and its orientationwith respect to the chain axis. The dependence of the dispersion curves on thespheroid aspect ratio leads to a number of interesting effects. In particular,bandwidth of SPPs that can propagate in an LPC can be substantially increasedby utilizing prolate or oblate spheroids. When q is close to a critical value,so that |q - q_c^\perp|<<\pi/h or |q - q_c^\parallel|<<\pi/h, the decay lengthof the SPPs is dramatically increased. In addition, the dispersion curvesacquire a very large positive or negative slope. This can be used to achievesuperluminal group velocity for realistic chain parameters. We demonstratesuperluminal propagation of Gaussian wave packets in numerical simulations.Both theory and simulations are based on Maxwell equations with account ofretardation and, therefore, are fully relativistic.

From Slow to Superluminal Propagation: Dispersive Properties of Surface Plasmon Polaritons in Linear Chains of Metallic Nanospheroids